The present invention relates generally to game balls, and more particularly to a method of coating game balls using a coating which cures rapidly.
For decades, golf balls were finished by applying one or more coats of paint or the like to the outer surface of a golf ball cover. To achieve a desirable white appearance, a multicoat paint system typically comprising a primer coat followed by one or more opaque coatings was applied to the golf ball. Such golf balls are typically referred to as xe2x80x9cpainted ballsxe2x80x9d. In the 1980""s, the concept of incorporating pigment into the cover material prior to molding the cover of the golf ball was introduced, thereby eliminating the step of painting the golf ball. Such golf balls are typically referred to as xe2x80x9cpigmented ballsxe2x80x9d.
Whether the golf balls are painted or pigmented, identifying indicia such as the manufacturer""s trademark or logo, or a model or identification number, are stamped on the ball. In order to prevent the stamped indicia from being rubbed off, and also to impart a desirable glossiness to the ball, one or two clear outer coatings are applied to the ball. Typically, such clear coatings consist of a clear solvent based primer or waterborne primer followed by a clear urethane top coat. The logo or other indicia typically is stamped on the primed ball before application of the top coat. The urethane top coat is usually a two-part polyurethane, consisting of a polyol and a polyisocyanate mixed together and reacted to form the urethane coating.
In golf ball manufacturing, the period of time required to apply and cure the final glossy top coat encompasses a large percentage of the total time required for golf ball manufacture. Known golf ball finish coatings typically require 6-8 hours of curing at an elevated temperature. Curing is generally conducted in a batch-type manner in large ovens.
One advantage of the conventional method for finishing golf balls is that the long cure time of the final top coat ensures that adhesion of the finish coating to the golf ball is strong, and as a result, the golf ball has a long, useful life. Another advantage is that the pot life of the mixed coating is relatively long. However, known methods are disadvantageous in that the 6-8 hour curing time prevents same day delivery of golf balls, particularly customized logo balls. Furthermore, one or more very large ovens are required if a sizable number of golf balls are to be manufactured daily.
In a conventional polyurethane finish coating system for coating golf balls, a polyol and a polyisocyanate are mixed in a pressure pot without a catalyst. The mixture in the pressure pot is then sprayed on the outer surface of the golf balls. Most of the mixture remains in the pot for some period of time before spraying. Thus, it is important to include enough solvent in the coating mixture to prevent the coating mixture from curing while it is still in the pressure pot and to provide for acceptable flowout of the coating mixture. However, it would be environmentally beneficial to be able to reduce the quantity of solvents in the coating system.
An object of the invention is to provide a method for applying a fast curing coating system to game balls, such as golf balls, softballs, baseballs, cricket balls and the like.
Another object of the invention is to provide a method of making a game ball with a fast curing coating that has adhesion and abrasion resistance which is comparable or superior to prior known urethane coatings.
Yet another object of the invention is to provide a method of making a game ball using a coating system which has a high solids concentration.
A further object of the invention is to provide a method for reducing the time required to cure a finish coating which is applied to the surface of a game ball.
Yet another object of the invention is to provide a method for coating a game ball using a low V.O.C. coating system.
Other objects of the invention will become apparent from the following description.
One preferred form of the invention is a method for coating a game ball. The method comprises the steps of providing an unfinished game ball, applying a fast-curing coating on the outer surface of the ball in an average initial wet thickness of no more than about 3.5 mils, and curing the coating at an elevated temperature for a period of time sufficient to substantially dry and cure the coating. Curing takes place at an elevated temperature for a period of about 5 minutes to 1.5 hours. Preferably, curing is at 110-180xc2x0 F. for about 5 mins. to xc2xd hour, and more preferably at about 150-170xc2x0 F. for about 5-15 mins. The coating used according to the method of the invention comprises a mixture of a polyol, polyisocyanate which is present in quantity appropriate to provide a ratio of xe2x80x94Nxe2x95x90Cxe2x95x90O to xe2x80x94OH in the range of 0.9 to 1.8 with up to 65 weight % of a solvent based upon the weight of the coating system, and a compatible catalyst. The catalyst is present in a quantity sufficient to provide for an at least 100% increase in viscosity of the coating within 30 minutes after mixing if the mixture is maintained at 25xc2x0 C., 1 atm and 60% relative humidity.
The catalyst preferably comprises at least one member selected from the group consisting of dibutyl tin dilaurate, dibutyl acetylacetonate, dibutyl tin dibutoxide, dibutyl tin sulphide, dibutyl tin di-2-ethylhexanoate, dibutyl tin (IV) diacetate, dialkyltin (IV) oxide, tributyl tin laurylmercaptate, dibutyl tin dichloride, organo lead, tetrabutyl titanate, tertiary amines, mercaptides, stannous octoate, potassium octoate, zinc octoate, diaza compounds, and potassium acetate. The catalyst preferably is present in a quantity of 0.01-10 weight % active catalyst (not including any carrier) based on total resin solids (polyol plus polyisocyanate, excluding solvents). The quantity of catalyst will depend upon the type of catalyst, polyol, polyisocyanate, and solvents which are used, as well as the curing temperature and desired curing time. When dibutyl tin dilaurate is used as the catalyst, it preferably is present in an amount of about 0.05-0.35 weight % active catalyst based upon total resin solids, and more preferably 0.08-0.15 weight % based upon total resin solids. Excellent results have been achieved using about 0.1 weight % dibutyl tin dilaurate based upon total resin solids. Other catalysts preferably are used at levels which will produce the same state of cure as 0.05-0.35 weight % or more preferably 0.08-0.15 weight % active dibutyl tin dilaurate. The catalyst preferably is present in an amount sufficient to reduce the curing time of the coating by at least about 75% as compared to a coating system which does not contain the catalyst but is otherwise identical.
The polyol preferably comprises at least one of a polyester, polyether or acrylic, and has a hydroxyl equivalent weight of 50 to 1500, and more preferably 100-1000. Blends of different polyols can be used. More preferably, the polyol is a saturated polyester polyol, and most preferably is the reaction product of an organic acid which includes at least one member selected from the group consisting of adipic acid, phthalic acid and isophthalic acid, an anhydride, and a glycol which includes at least one member selected from the group consisting of ethylene glycol and trimethylol propane.
The polyisocyanate preferably includes at least one member selected from the group consisting of biurets and isocyanurate trimers of hexamethylene diisocyanate. Blends of polyisocyanates can be used. The solvent preferably includes one or more members selected from the group consisting of ketones, esters and acetates. Another preferred form of the invention is a method of coating a game ball comprising the steps of providing an unfinished game ball, applying a fast curing two-part coating system to the surface of the unfinished game ball in an average initial wet thickness of no more than about 3.5 mils, the coating system comprising a compound containing an active hydrogen atom, a polyisocyanate, and a compatible catalyst, the compound containing an active hydrogen atom and the a polyisocyanate being combined no more than about ten minutes prior to application, and curing the coating at an elevated temperature until the coating is substantially dry, the curing process taking no more than about 1.5 hours.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements exemplified in the following detailed disclosure.